Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) systems are used to obtain and analyze x-ray emission spectra from mineral samples in order to determine sample composition. A mineral sample is raster scanned via an electron beam, and x-ray emission spectra are recorded on a per pixel basis and analyzed to assign a single mineral to each pixel. Two currently existing methods used to identify and assign a mineral are outlined here. In the first, the sample x-ray emission spectrum is compared to single mineral x-ray emission spectra stored in a library, and a mineral with the closest matching x-ray emission spectrum is assigned to the pixel. In the second, the sample x-ray emission spectrum is fit to a linear combination of elemental x-ray emission spectra stored in a library. A least squares algorithm is used to determine the percentage of each element in the combination and how well the spectrum of the linear combination fits the sample spectrum. The elemental percentages are then used to identify the closest matching mineral (e.g., one having the same elements and percentages) from a catalogue of mineral definitions, and that mineral is assigned to the pixel. Other methods include integrating energy regions to obtain a photon count associated with an element; and matching a collection of integrated photon counts against a catalogue of mineral definitions. The energy ranges may be as narrow as 1 channel, or as wide as the peak of an EDS signal.
The methods described above adequately identify sample composition on a per pixel basis when the grain size of minerals in the sample is larger than the pixel size of the raster scan. If the grain size of minerals in the sample is smaller than the pixel size, the pixel size can be reduced by increasing the resolution of the SEM. However, a natural limit is reached when the pixel size becomes smaller than the x-ray interaction volume or the volume within which x-rays are produced in the sample at a given SEM resolution. At this limit, the measured signal originates in a physical volume in the material that cannot be reduced by further reducing the pixel size or increasing the SEM resolution. For conventional beam energies of 15 keV-20 keV the volume is on the order of 1 μm-3 μm, and this occurs in certain common and economically important mineral samples, such as shales, marls and laterites.
In fine grained mineral samples, two or more minerals can contribute to the recorded x-ray emission spectrum because the mineral grains are smaller than the x-ray interaction volume. Although this volume is a direct function of the accelerating voltage of the electron beam, it cannot be reduced because doing so would require generating an electron beam having insufficient energy to excite x-ray emissions of the elements in the sample. This beam voltage requirement therefore limits the minimum usable beam voltage to a range of approximately 10 kV-20 kV. At these beam energies, the clay mineral grains in a shale sample are often much smaller than the x-ray interaction volume. As a result, the closest matching single mineral spectra returned by either of the previously described known mineral analysis techniques can be inaccurate or incomplete.
The least squares method used to fit a combination of elemental spectra to the sample spectrum has also been used to fit a combination of mineral spectra to the sample spectrum. While this technique, in principle, could allow for sub-pixel mineral identification in samples that are non-homogeneous at the scanned pixel level, it is difficult to resolve the contributions of different minerals to the overall sample spectrum since many mineral spectra have overlapping peaks. This makes it extremely difficult to determine the relative proportion of the elemental x-ray signals that are due to different minerals within the pixel